Bob Costas is getting a new substitute on NBC's Olympics broadcast on Friday. It's Meredith Vieira.

Costas will be sidelined for a fourth day as NBC's prime-time host because of an eye infection, and Matt Lauer filled in for him three days in a row. But with Lauer also anchoring the "Today" show -- and making himself available for stunts like a luge ride on Friday's show -- Olympics producer Jim Bell says he needs a break.

Bell said Friday that Costas is improving, but for someone with an infection that makes him sensitive to light, a television studio is a lousy place to be. Vieira, who was Lauer's former co-host on "Today," co-hosted the opening ceremony with him last week.

It's the first time a woman has ever solo hosted Olympics primetime coverage, according to NBCOlympics.com